getenv_strict needs to be based on getenv (accidentally broken in 0e88d33e8d19);

--- a/src/Pure/System/isabelle_system.scala	Fri May 22 13:22:16 2009 -0700
+++ b/src/Pure/System/isabelle_system.scala	Sat May 23 17:21:44 2009 +0200
@@ -27,7 +27,7 @@
   }
 
   def getenv_strict(name: String) = {
-    val value = environment.get(name)
+    val value = getenv(name)
     if (value != "") value else error("Undefined environment variable: " + name)
   }